test: reproduce possible Hash inconsistency when [] / []= concurrently

[kares]

... a sometimes reproducer for #51 (needs a GIL free Ruby such as JRuby)

so that shipping #352 is justified // cc @radar

[kares]

NOTE: it might not always fail, the outcome is simply non-deterministic

... for me, on JRuby 9.1.13.0, the failure shows up at least once in ~ 5-7 runs :

$ ruby -w -I"lib:lib:test" -I"/opt/local/rvm/gems/jruby-9.1.13.0@temp/gems/rake-12.1.0/lib" "/opt/local/rvm/gems/jruby-9.1.13.0@temp/gems/rake-12.1.0/lib/rake/rake_test_loader.rb" "test/backend/memoize_test.rb" 
test_declarative is deprecated for Minitest::Unit::TestCase
test_declarative is deprecated for Minitest::Test
Run options: --seed 57362

# Running:

.....................{:foo=>{:"scoped.sample"=>"[:foo, :scoped, :sample]"}, :bar=>{:"scoped.sample"=>"[:bar, :scoped, :sample]"}, :baz=>{:"scoped.sample"=>"[:baz, :scoped, :sample]"}}
F..........

Finished in 0.716656s, 44.6518 runs/s, 46.0472 assertions/s.

  1) Failure:
I18nBackendMemoizeTest#test_lookup_concurrent_consistency [/home/kares/workspace/oss/i18n/test/backend/memoize_test.rb:78]:
NON-THREAD-SAFE lookup memoization backend: Hash.
Expected: 3
  Actual: 4

[radar]

LGTM. Thanks for your work on this!